When to shift a 2004 Boxster S

[Anker]

This is fun.

I am off to a HDPE tomorrow and would like to know the ideal, or close to ideal shift points for maximum acceleration

The information needed to establish this is in the owners manual in the performance graphs in the back.

The first thing is to establish some torque bands using the torque/RPM chart.



I have marked the maximum torque point with a green line, the ideal torque band bracketed with yellow lines and the band you don't want to go outside with red lines.

Why they are placed where they are should be pretty obvious. After 6000 RPM the torque drops off dramatically and I have placed the other red line at the same torque value. The yellow is simply at the torque values half way between the ideal and the red.

The bands are then transferred to the RPM vs speed chart.



So now we have to look at the RPMs to shift at to be at the optimal torque bands.

Starting with first gear I draw a blue line and look for a point where the band between the first gear line and the second gear line sits in the best possible torque band. Because these two gears are spaced pretty far apart you can't avoid falling outside the red band. But we definitely don't want to exceed 6000 RPM, so that's where we shift.

In second gear we can stay inside the red band if we shift at 6000 RPM again. We could shift a tad earlier and possibly get a tiny performance improvement, but 6000 is easy to remember, so that's where I shifted from second to third.

Spacing between third and fourth is close enough to make it easy to stay comfortably inside the yellow band. Shifting at 5250 RPM puts us nicely around the max torque

Exactly the same happens when shifting from fourth to fifth and fifth to sixth at 5250 RPM.

So the answer is simply. When looking for maximum acceleration, upshift from first and second at 6000 RPM and from third, fourth and fifth at 5250 RPM.

Downshifting points also fall out of the same chart. You should downshift from sixth, fifth, fourth and third at roughly 4250 RPM and from second at 3500. I am going to mark 6000, 5350, 4250 and 3500 om my tach with some stickies.

Off to the track.

[The Radium King]

except ...

torque at the wheel = engine torque x gear ratio.

so, to calculate your shift points you have to take the engine torque vs rpm data and multiply by gear ratios. do that and you'll see that even with the drop-off in engine torque past 6000 rpm, you are still making more torque at the wheel than if you shifted to a higher gear. shift at redline.

[Anker]

That is already in the charts. The slopes of the lines on the second chart reflect the gear ratios!

[JayG]

How much track experience do you have?

Simple
upshift 6500-7000
If you are shifting below that you are short shifting and will have slower lap times
you definitely don't want to be below 4000, no power

torque gets you going and HP keeps you going
wind it out, the rev limiter will protect you on upshifts.

A lot of where you shift has to do with your speed and what the next corner is and how close to the corner you are. Remember that shifting causes weight transfer. You don't want to up-shift because of some chart and a second later downshift because of the corner

[The Radium King]

say max engine torque is 310 nm at 4500 rpm, while engine torque at 7000 max rpm is 250 nm.

gear ratios on a 6-speed are as below, followed by wheel torque calcs at max engine torque and max rpm ...

1 3.82 1185 955
2 2.20 680 550
3 1.52 470 380
4 1.22 380 305
5 1.02 315 255
6 0.84 260 210

from this you can see that it is better to stay in first to redline (955 vs 680) and second to redline (550 vs 470) and third to redline (380 to 380, but note the time and power loss during a shift). arguably fourth to fifth and fifth to sixth could be short shifted a bit, but that is a fast track if you are running in those gears. also note that final drive, as well as wheel and tire radius, have not been included in the calcs.

[steved0x]

I like to shift at 6,000-6,300'ish not for power purposes but for (real.or imagined) engine longevity. If a turn is coming up I might wind it outva bit instead of going 3->4 then right back to 3.

One thing for sure, JayG is right, don't fall below 4000 , you will wonder what happened...

I would respectfully submit that there is no way to be looking down at the tach for every shift , if I did I would be all over the road...

Once you get a few laps you will have your shift points figured out.

Final.thought: Have fun and post a trip report!

[AZ986S]

Finally! Data (The Radium King) that backs up what I feel in the seat of my pants at the track It is very clear from those numbers that, for max acceleration (and lowest lap times) you need to get as close as possible to redline when shifting from 1st to 2nd to 3rd , and also possible 4th. You will see that you loose time at every shift, and coasting (not accelerating) also makes you loose time but may be necessary if you are shortly about to enter a braking zone VS shifting into 4th then back into 3rd 1 second later, more lost time from the shift than lost time from the brief coast at 7,000 rpm.

Engine longevity is a different issue and I tend to agree with Stevedox as I try to shift a little early (6,500-6,800) as you might peg redline and then some if your shifts aren't precise. Rev limiter should reduce that occurrence, but I have learned that doesn't always happen If I know I will have to shift to 4th or higher, I will shift a little early maybe 6,000-6,500 just to save some wear and tear...

Enjoy your HPDE! And beware of the money shifts!

[NewArt]

Originally Posted by JayG

A lot of where you shift has to do with your speed and what the next corner is and how close to the corner you are. Remember that shifting causes weight transfer. You don't want to up-shift because of some chart and a second later downshift because of the corner

Like Jay says, as you get to know a track, you will know when to shift, regardless of what the charts say. The power band is between 4 and redline but you will make your time in the corners!

[Anker]

I realize that Jay is right that the numbers have to be adjusted by the difference in gear rations. Just for the fun and the heck of it I'll keep massaging the numbers.

[jakeru]

Analysis fail. You'll want to maximize the area under the horsepower curve. When you have the dyno plots (hp / torque vs rpm for range) like you do, you can essentially ignore torque vs rpm for this exercise.

If you are upshifting (in any gear) before even reaching the horsepower peak at ~6200 rpm, you're definitely upshifting too early.